Topic: Is a fast idle bad??

[jonnycbgood]

I have a 81 750K...I know it is not a SOHC but you guys are just so smart I need your opinion.  I have been working on the carbs and what not.  Everything looks good and I did a sync on the carbs.  The problem is that I cant get it to maintain an idle below 2000RPM.  I can get it down to 1500 but it ends up dying.  Is a idle at 2000 bad will it do any damage?   

There are quite a few mods on this bike.  Mikuni VM29 carbs, Kerker 4-1 exhaust, Ignition coils wired direct from battery Not sure if any mods have been done to the cam or heads.  But it is definitely set up for speed and i am wondering if this is the cause of the fast idle?

Thanks for the help

[ieism]

Can you improve it with the airscrew on the carbs?

It's probably the aftermarket carbs, or you may have a non stock cam in your engine.

As long as it doesn't overheat, I wouldn't worry about it too much. Change your oil more often if the it runs hotter that it should. 

[jonnycbgood]

I didnt really see any change when I was moving the air screws.  Like your supposed to see it increase and then decrease and set it in the middle.  I could see any change...I was thinking that it might have a aftermarket cam.  Thanks for your thoughts.  What is the best way to tell if the engine is overheating??  With no temp gauge?

[ieism]

That's hard to say as I don't know your bike. It's aicooled so if you ride in the city a lot, with stop/go traffic the engine will get hot. Any ticking pinging noises when it cools down? If it gets worse in hot weather you could run different oil depending on the season.

Don't worry about it, just ride it.

[jonnycbgood]

excellent idea! that was my gut feeling but just wanted a second opinion.

Thanks

[heffay]

clean up your idle circuit on your carbs, then check for air leaks in the carb boots.
wiring your stuff back to stock may help quite a bit as well.




my best suggestion is don't settle for one member's suggestions... many, if not all, of us will have different things to say.   

[Aaron J Williams]

I didnt really see any change when I was moving the air screws. 

That would make me suspect a vacuum leak. You could try plugging a vacuum gauge into the port which feeds your fuel petcock and see what it does as you idle the bike down. If the needle swings wildly just before it dies then it is possible your motor is cammed. If it stays relatively steady then you may have a vacuum leak. What do the spark plugs look like?

[jonnycbgood]

the petcock is a direct feed not vacuum feed so not sure I can do that last test.  The idle circuits have been soaked in carb cleaner and cleaned thoroughly.  The plugs look normal lightly tanned.  I did notice that when I was syncing them the vacuum tended to drift a little could that indicate a vacuum leak.  I inspected the carb holders and didnt see anything.  I also tried spraying WD40 on them as the bike was idling and didnt see any change.  What other test could I do to find a vacuum leak?

[WHALEMAN]

I just worked on a friends 1980 CB750 and the intake manifold rubbers were cracked and leaking and I changed them and the idle and easy starting came back. They were so hard I had to cut them off the head. I could not pull them off even with the carbs off. Dan

[Aaron J Williams]

the petcock is a direct feed not vacuum feed so not sure I can do that last test.  The idle circuits have been soaked in carb cleaner and cleaned thoroughly.  The plugs look normal lightly tanned.  I did notice that when I was syncing them the vacuum tended to drift a little could that indicate a vacuum leak.  I inspected the carb holders and didnt see anything.  I also tried spraying WD40 on them as the bike was idling and didnt see any change.  What other test could I do to find a vacuum leak?

Just use your synchronizer as the vacuum gauge. Also, I would use carb cleaner to check for leaks instead of WD-40. Carb cleaner will raise the idle speed when a leak is found, I'm not sure if WD will. You need to have the bike idled down as far as you can before spraying because at 2K rpm you might not see a change in idle speed with the spray. Finally, if your idle jets are plugged, carb cleaner won't always clean them. You may have to remove the jets and poke the crud out of them with a tiny piece of wire and then flush with carb cleaner.

[eldar]

Do those carbs have any vacuum diaphragms? I think the stock 650 carbs and there for stock 750 carbs in those years had them. I know you have aftermarket but do they have anything like that?

[johnny_from_bel]

Just use your synchronizer as the vacuum gauge. Also, I would use carb cleaner to check for leaks instead of WD-40. Carb cleaner will raise the idle speed when a leak is found, I'm not sure if WD will. You need to have the bike idled down as far as you can before spraying because at 2K rpm you might not see a change in idle speed with the spray. Finally, if your idle jets are plugged, carb cleaner won't always clean them. You may have to remove the jets and poke the crud out of them with a tiny piece of wire and then flush with carb cleaner.

WD-40 will raise idle speed. don't worry.

[jonnycbgood]

with the idle jets I used a wire off a wire brush to poke through them along with the carb cleaner,

these are mechanical throttle slide carbs I did not see anything resembling a vacuum diagphram

I will try the carb cleaner next on the boots and see if that does anything.

One more problem I discovered today...I suspect that my charging system is not working properly. So I checked the voltage across the battery with the bike off 12.7 volts ... I then checked it again with the bike idling and got 0 volts? can anyone direct me to an electrical guru so I can figure out what I should check next?

[Aaron J Williams]

Man! You are having some poor luck with this one Johnny. It seems to me that the usual suspect in the DOHC charging system is the alternator rotor. If I recall correctly it should have 3.5 ohms resistance or more to be functional. I seems quite odd to me that a bike which is electric start only would have enough juice to start the bike but read 0 volts while running. Even if the charging system is totally wasted you should see some voltage at the battery or else the bike would have no power to fire the ignition. Is your bike named Christine?

[jonnycbgood]

Yeah it has me baffled. I have some time off this next week so i plan to check each part of the charging system very carefully.  And you are right about the rotor although I have my fingers crossed that it is just brushes.  12 vs 100 dollar fix.  My battery is on a trickle charge so I was fully charged but after taking it out for 15 min it only had 11.5 volts and wouldnt start.  But regardless I dont know how you can not get any voltage across the battery. 

When I got the bike from the PO it had a dead battery and he said he couldnt get it started.  So I started with a new battery and plugs which did great.  Then worked on the carbs to get the idle right which I am still working on that and now I see why the battery was dead.

[TomC]

Hi jonnycbgood
     Before you start buying parts. Clean as many electrical connections as you can. I would start by cleaning the connections at both ends of the bikes battery cables.
     If you actually have Zero volts at the battery with the engine running you have (a) found your idle problem (b) a good working charging system. But the only idea that I have on how that could happen is an intermittent open inside the battery. Which I have never heard of happening. I am assuming were measuring voltage at the battery terminals.
          TomC in Ohio

[heffay]

But regardless I dont know how you can not get any voltage across the battery. 

read your multimeter instructions... i'm betting you had it on the wrong setting.

[jonnycbgood]

I am going  battery post to battery post and I have double and triple checked the settings just because I dont believe it to be possible.  When the bike is off I test across the battery and get >12.5 volts and then start the bike and test in the exact same location and manner and get nothing.

I am seeing it and I dont believe it

[heffay]

well, u asked for guru and i'll give it to ya.... ask twotired.

your predicament is weird... i'll give ya that, for sure!

[TomC]

Hi jonnycbgood
     Are you using the electric starter when you start the bike? Do DOHC4s have kick starters?
     If the bike is running there needs to be I am guessing over 10v going to the ignition system. I am assuming that your DOHC4 has an electronic ignition system which I suspect needs at least 10v to work. Are your head light and tail light on when the bike is running?
     On a SOHC4 the negative terminal of the battery is grounded to the engine at one of the rear engine mounting bolts. The positive terminal of the battery is connected to one of the terminals of the starter solenoid as is the output from the rectifier and the wire going to the main fuse.  My guess is your DOHC4 is the same.
     I would attach the negative lead of your volt meter to some were on your engine. Having done that I would measure the voltage at the negative battery terminal, the positive battery terminal, and at the post on the starter solenoid with the every thing off. Then measure again at the same places as you try to start the bike. Then with the bike running. With the engine running and hold about 3,000 rpm and measure at the starter solenoid for maybe 15 - 30 seconds and see if the voltage changes. Does it go up or down? Then report back.
          TomC in Ohio

[jonnycbgood]

TomC

Okay there is no kick start only electric and it has to have greater than 11V to be able to get it started.  I have the ignitio ndirect wired via a relay from the battery.  All lights are in working order.  I put the key on and turn the headlights on high and check voltage = 11.5V  Yes the battery is grounded to the frame and the postive goes to the starter solenoid where I tapped off for my direct wire.  There is a 30 amp fuse here that I believe is the main fuse for the bike.  The R/R plugs in here with four wires.  With the bike off

Batt '-' 0V     Batt '+' 13.39V     Starter solenoid 13.39V

With the bike idle at 2K

Batt '-' 0V - >20V
Batt '+' 0V - >20V
Starter 13V - >20V

I think this means the regulator is bad.  I am going to check the resistance across the rotor and check the yellow wires coming from the alternator.  Anything else to check?

[TomC]

Hi Jonnycbgood
     Tell me more about how you have you bike's ignition wired. I think that this may be a big part of your problem. I have a CB1000C wiring diagram on my computer. I will spend some time looking at it and thinking.
          TomC in Ohio

[TwoTired]

TomC

Okay there is no kick start only electric and it has to have greater than 11V to be able to get it started.  I have the ignitio ndirect wired via a relay from the battery.  All lights are in working order.  I put the key on and turn the headlights on high and check voltage = 11.5V  Yes the battery is grounded to the frame and the postive goes to the starter solenoid where I tapped off for my direct wire.  There is a 30 amp fuse here that I believe is the main fuse for the bike.  The R/R plugs in here with four wires.  With the bike off

Batt '-' 0V     Batt '+' 13.39V     Starter solenoid 13.39V

With the bike idle at 2K

Batt '-' 0V - >20V
Batt '+' 0V - >20V
Starter 13V - >20V

I think this means the regulator is bad.  I am going to check the resistance across the rotor and check the yellow wires coming from the alternator.  Anything else to check?

Where are you placing the probes for these tests?
The meter should be on the DC setting.  It reads voltage potential across the probe tips.  The battery cannot actually have zero volts across the terminals unless it is stone cold dead.  If it is not dead, then your meter or test technique is faulty.

Because of the variation in the way digital meters are implemented.  I suppose it is possible that shorted diodes in the rectifier may be placing AC voltage on the battery terminals, an these voltage spikes are sending your DC reading off scale.
If you have faith in your meter, then change the scale to an AC range, and note the reading.  There should not be much of a reading.  If you see 30-60 volt peaks, then the rectifier is suspect.
I don't know if you have a combined regulator/rectifier, or separate units lie the SOHC4's had when stock.  An AC voltage read with a DC scale would read 0V.  Feeding AC voltage to a battery will kill it eventually.

I was reluctant to respond to this post, simply because it is forum abuse.  It belongs in other bikes.  I will report this to the mods.

[TomC]

Hi jonnycbgood
     I agree with TwoTired that this thread should be in Other Bikes. I am going to reply in Other Bikes under the title DOHC Electrical Weirdness.
     TomC in Ohio

[jonnycbgood]

TwoTired,

I apologize for placing the posting in the wrong board thanks for correcting it.  The last test I grounded the common probe to the engine block and used the red probe and a spot tester.  The meter was set to 20V DC.  To tell you the truth I dont have a lot of faith in my meter seeing as I only paid 5 bucks for it.  I am going to borrow one from a friend that is calibrated and more accurate.  With that being said I tested the resistance across the copper windings of the rotor and got ~ 5.5 ohms.  i then unplug the stator from the R/R it had 5 wires - 3 yellow, 1 black (hot), 1 white (neutral).  i tested the resistance across the black and white and got off scale reading.  Across any of the three yellow wires was about 1.3 or so.  The housing and stator was covered with magnectic dust particles and the plugs is pretty dirty I am going to work on cleaning all the connections and the housing.  And then use a calibrated meter to see if any of the results change.  Any other ideas I am all ears?